This invention relates generally to a device for controlling the operation of an air conditioning system, and more particularly, to an electronic control circuit for a variable displacement compressor in an automobile air conditioning system.
In conventional air conditioning systems known in the prior art, the air conditioning compressor is driven by a motor, either directly or through a magnetic clutch. Thermal control in the room or compartment is accomplished by the intermittent operation of the magnetic clutch or motor by a signal from a thermostat disposed in the room. Once the temperature in the room is lowered to the desired level, the supplemental cooling capacity of the air conditioner need not be as large for maintaining the desired temperature in the room. Therefore, after the room has been cooled to the desired temperature, conventional air conditioning compressors are intermittently operated in response to the thermostat signal. Compressors having large cooling capacities are operated even more intermittently because of the high amounts of energy required to drive such compressors.
In an air conditioning system for automobiles, the compressor is usually driven by the automobile engine. Because the r.p.m. of the automobile engine changes continuously, the rotation frequency of the compressor changes accordingly, resulting in rapid changes in the cooling capacity of the air conditioner. Automobile air conditioners are generally designed so that when the compressor is driven by the engine at normal driving speeds, the air conditioner operates at optimum capacity. Therefore, when the compressor is driven by the engine at lower operating speeds, or when the engine is idling, the cooling capacity of the air conditioner is insufficient to maintain adequate cooling of the vehicle. Conversely, when the compressor is driven at high operating speeds by the engine, cooling capacity is more than necessary.
One known device for controlling the excessive cooling capacity of air conditioning systems operates by reheating some of the cooled air generated by the air conditioner. Part of the air from the evaporator of the air conditioner is reheated in a heating unit. The reheated air is mixed with the remaining cooled air to control the temperature of the air which is ultimately blown into the room or compartment. In this device, the ratio of cooling capacity to heating capacity is controlled in response to the r.p.m. of the automobile engine. The above-described device for controlling air temperature is complicated and considerable energy is wasted in reheating the cooled air.
In another known device for controlling the cooling capacity of an air conditioning system, the magnetic clutch connecting the compressor to the driving engine or motor is intermittently operated to control the operation of the compressor. However, because the magnetic clutch is often required to engage the driving engine at high operating speeds, sudden and deleterious forces are generated at the moment the clutch is engaged and are transmitted to the engine and compressor. The temperature of the air which is blown into the room or compartment also drastically changes upon engagement of the clutch and operation of the compressor.
Referring to FIG. 1, there is shown a graph of the temperature variations of the air blown by a conventional air conditioning system. When the air conditioning system is first turned on and the compressor begins to operate, air having relatively high temperature Tth is blown into the room or compartment. As the compressor continues to operate, temperature Tth gradually falls to predetermined temperature T.sub.2. At this point, the compressor stops operating and temperature Tth begins to rise. When temperature Tth reaches predetermined temperature T.sub.1, the compressor begins operating again. Intermittent operation of the compressor continues as described above so that the temperature of the blown air is kept within the range T.sub.1 -T.sub.2, as shown in FIG. 1.
The rate of operation K.sub.0 of the compressor required to maintain the temperature of the blown air within the range T.sub.1 -T.sub.2 may be determined by the equation K.sub.0 =t.sub.1 /(t.sub.1 +t.sub.2), where K.sub.0 is the rate of operation of the compressor, t.sub.1 is the length of time that the compressor is in an operating state and t.sub.2 is the length of time that the compressor is in a non-operating state. When the rate of operation of the compressor is less than 1, the refrigerating or cooling capacity of the compressor is in excess of what is actually required by the cooling load, i.e., in excess of what is required to maintain the temperature of the blown air within the range T.sub.1 -T.sub.2. Thus, a large amount of energy is wasted in generating cooling capacity that is not needed. Contrarily, when K.sub.0 is close to 1, the compressor is operated at maximum efficiency to satisfy the required cooling load and little energy is wasted.